How Seriously to Take Terror Alerts?

Spencer Ackerman wrote a bit about the alert system that has replaced the color-coded system of the 9/11 decade. He quotes Bruce Schneier on how much precious energy we should devote to the terror threat:

“I can quantify it really easily: not at all. Zero,” [Schneier] says. “You should be zero scared of terrorists. And maybe 1/120th more afraid of terrorists than you are of everyone else.”

Do public narrative shifts like this always take a full decade? I’d be interested to see how 9/11 compares to topics of similar weight.